Despite significant progress in recent years in the prevention of dumping, and the removal of contaminating materials from the soil and ground water, it remains a major environmental problem in the United States and other industrialized nations. In many areas of the country, the magnitude of subsurface contamination poses a serious threat, both to the health of humans and wildlife near it, and to the environment as well. While governmental regulations have mandated the control and reduction of such subsurface contamination, the state of the technology available to clean-up contamination sites has often lagged far behind, and when such technology has proven effective it has also usually been prohibitively expensive.
Soil Zones Which Carry Contaminants
With regard to the subsurface soils which form the contamination sites with which the present invention is concerned, this ground water environment is generally divided into two major zones, (1) the unsaturated zone, also known as the "vadose zone" and (2) the saturated zone. However, perched water zones are also included, although the vadose zone is the most important. The vadose zone extends from the ground surface down to the ground water table, while the saturated zone begins at the ground water table and extends to a further depth. The vadose zone may be further divided into additional subzones, but will be treated as a single zone in the present discussion. Since the vadose zone is the uppermost layer of the terrestrial environment, it contains the most important pathways for the toxic and hazardous chemicals to enter groundwater systems. As such, the removal of the toxic and hazardous chemicals in this zone is of paramount importance for all groundwater remediation. The principal mechanisms that control the flow and transport of contaminating chemical compounds in the vadose soil zone are mass flow, liquid diffusion, and vapor diffusion. Transport of those contaminants from the unsaturated to the saturated zones occurs continuously by percolation and vapor transport, thus making treatment of the vadose zone essential to any successful ground water remediation.
In Situ Remediation--Vapor Extraction
Moreover, where methods have been developed in the past which utilize wells to extract ground water samples from beneath the surface and bring those samples above ground for remedial action, it has been the case that the equipment for effecting groundwater clean-up has been normally complex and often only marginally effective. Indeed, studies have shown that it is less costly to remove volatile organic compounds (VOCs) from the vadose zone than to pump and treat contaminated ground water. It is for that reason that technology is currently being developed for the in situ removal of VOCs from the vadose zone. Such treatment technologies include vapor extraction, biodegradation, soil washing and thermal treatment. Vapor extraction is a process for the in situ removal of VOCs by mechanically extracting soil gases from the vadose zone. Specifically, one or more vertically oriented perforated vent wells are installed in the contaminated zone in the ground, and air is forced to travel through the pore space in the soil, causing volatilization of the liquid and adsorbed volatile organic compounds. The extracted soil gases are then either vented to the atmosphere or into an emission control system, depending on the concentration. The two major embodiments of such vapor extraction processes which have been demonstrated successfully in field use are in situ air stripping and vacuum extraction.
In order to carry out in situ air stripping, a series of interconnected air injector vents are supplied with forced air by an above ground blower and manifold system that forces the air into the soil through the perforated vent wells. A separate blower and manifold system is used to apply negative pressure to air extraction vents to withdraw the soil gases. The injection and extraction vents are located alternately within the array of vent wells on the site. To achieve a degree of flow containment, extraction vents are placed on the perimeter of the area being treated. However, this approach functions best with highly permeable soils, e.g., loose, sandy soils and has proven to be much less effective in tightly packed soils and in soils with a high clay content. Vapor extraction processes involving vacuum extraction utilize a vacuum pump installed on the wells which induces a negative pressure gradient around the well to remove the VOCs along with the soil gases.
In Situ Remediation--Biodegradation
Biodegradation is another process which has been used effectively in the treatment of soils contaminated with organic compounds. In the biodegradation process, usually referred to as bioremediation, the ecological conditions in the soil are altered to enhance microbial catabolism or to cometabolise the organic contaminant, thus transforming it into a simpler, non-toxic product. In most applications, indigenous microorganisms are utilized, although seeding of the soil with exogenous microorganisms has also been used where naturally occurring microorganisms are unable to degrade the contaminants Microorganisms are either (1) aerobic, which grow only in the presence of oxygen, (2) anaerobic, which grow only in the absence of oxygen, and (3) facultative anaerobic, which can grow either in the absence or presence of oxygen. The biodegradation method which has been found most effective in treatment of the vadose soil zone has been the aerobic microbial process. With this process, oxygen and often nutrients are injected or infiltrated into the subsurface environment, using wells or a percolation process. For example, wells are drilled into the soil and nutrients for feeding the microbes are dropped down into the well, or microbes are seeded in the well. Thereafter, the microbes are blown outwardly by forced air or the like. A concise summary of the major factors which affect the rate of biodegradation in the vadose zone are described by R. L. Valentine et al. in "Biotransformation", Vadose Zone Modelling of Organic Pollutants, eds. Stephen Hern et al., Lewis Publishers, Inc. Michigan, Chapter 9 (1986), and include: pH, temperature, water content, carbon content, clay content, oxygen, nutrients, the nature of the microbial population, acclimation and concentration. While a number of investigators have reported successful application of in situ biodegradation, important limitations exist, such as unfavorable reaction kinetics, low substrate concentration and slow degradability of certain compounds.
A number of specific applications of bioremediation with different degrees of success have been reported in the literature. R. Wetzel et al. in "Demonstration of In Situ Biological Degradation of Contaminated Ground Water and Soils", Sixth National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1985) describe a demonstration at Kelly Air Force Base, Texas, treating contaminants comprising of hydrocarbons, aromatics and halogenated organics, but noting that a major limiting factor of the remediation was the low permeability of the fine-grained soil layers present at the site. V. Jhaveri et al. describe in "Bioreclammation of Ground and Ground Water by In Situ Biodegradation" "Case History", Sixth National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1985) report the bioreclammation of a New Jersey site contaminated with methylene chloride, n-butyl alcohol, acetone and dimethylaniline, where after three years of in situ aerobic biological treatment, the contaminant plume was reduced by 90%. P. Yaniga et al. in "Aquifer Restoration Via Accelerated In Situ Biodegradation of Organic Contaminants", Seventh National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C. (1986), describe the reclamation of an aquifer contaminated with benzene, toluene, and xylene using biodegradation, and emphasize the importance of oxygenating the subsurface environment, reporting superior rates of biodegradation using hydrogen peroxide as an oxygen donor, compared to using the traditional technique of air sparging.
Known Methods and Apparatus for Carrying out In Situ Bioremediation
A method and apparatus for establishing, maintaining and enhancing microorganisms utilized to remediate groundwater or soils contamination through the injection of nutrients and gases, using a cylindrical head with radial apertures and a pointed lower end adapted to penetrate the soil, and through which a fluid can be delivered, is disclosed in Albergo et al. U.S. Pat. No. 5,133,625. The fluid, which may be a viable microorganism culture containing nutrients, or may be a gas which permits or enhances the growth of ambient microorganisms, is introduced into a subsurface location under pressure through the apertures in the cyclindrical head. The pressure is provided by a pump or other means, and is adjustable. However, nothing in the disclosure of Albergo et al. suggests the method and apparatus of the present invention and the dramatic results achieved thereby. In Billings et al. U.S. Pat. No. 5,277,518 it is suggested that an oxygen-containing gas can be used to provide microorganisms and nutrients to the subsurface, and that injection wells can be connected to an air compressor for this purpose. In Payne et al. U.S. Pat. No. 4,945,988, a sparging process and apparatus is modified by placing an oxygen separator along conduit lines leading to an aquifer downstream of an air pump, which permits the delivery of air which is substantially oxygen free to the aquifer, or is oxygen enriched to the vadose zone, thereby preventing growth of aerobic bacteria in the aquifer, while stimulating such growth in the vadose zone. However, there is no suggestion in either disclosure of pneumatic fracturing or of the other novel features of the present invention.
Known Method and Apparatus for Pneumatic Fracturing of Soil Formations
Paramount among the limitations of the existing and emerging treatment technologies applicable to the vadose zone is the permeability of the soil formation being treated. The efficiency of in situ treatment processes all decrease as the soil permeability decreases. For soils with low permeabilities the existing processes are largely ineffective. Low soil permeability may be caused by a number of factors, including high clay content, high soil density and high fluid viscosity. Therefore, the effectiveness of virtually all in situ treatment processes in the vadose zone can be enhanced by increasing the permeability of the soil formation. Indeed, an important advance in this area was made by Schuring et al. with the discovery that pneumatically fracturing the contaminated soil formation leads to a significant improvement in the results obtained with a variety of in situ decontamination methods. The details of that discovery and the method and apparatus which are used to implement it are disclosed in Schuring et al. U.S. Pat. No. 5,032,042, which is incorporated herein by reference it its entirety.
The method described by Schuring et al. for eliminating subsurface contaminants from soil includes the steps of a) pneumatically fracturing the soil, including the steps of i) inserting a tubular probe partially into a well in the soil such that at least one orifice of a nozzle fluidly connected with the tubular probe is positioned at a predetermined height; ii) providing a sealed area in the well on opposite sides of the orifice; and iii) supplying a pressurized gas to the tubular probe which travels through the orifice into the soil to produce a fractured soil formation; and b) transforming the contaminants into a different state to decontaminate the soil, after creation of the fractured soil formation. There is also described therein in general terms the use of such pneumatic fracturing to enhance microbial activity, particularly in terms of increasing oxygen concentration in the soil and thereby encouraging more rapid aerobic digestion of the soil contaminants, of reducing the water content of the soil to enhance biodegradation, and of providing nutrient seeding. However, there is no disclosure in Schuring et al. providing details of the apparatus and methods which might be used to achieve those objectives. The present invention by contrast, does provide such detailed disclosure, as well as a description of the guidelines for those methods and apparatus based on the mass transport and biological mechanisms by which the contaminants are degraded within the situs of the geologic formation. Indeed, using the apparatus and methods of the present invention it has been found that, surprisingly, a synergistic effect can be obtained with respect to the subsurface biodegradation processes, with a correspondingly increased rate of contaminant removal which is dramatic. In accordance with the present invention, it has been possible to extend bioremediation into low permeability geologic formations which are not treatable with current in situ remediation methods.